Angular displacement measuring systems serve to measure rotational movements of a shaft and are often also referred to as angle measuring means, angular displacement sensors, or rotary encoders. They are in particular used to control and monitor machines and vehicles. Contactless angular displacement measuring systems, for example, electrically or magnetically induced systems, here play a special role since they have a long service life due to wear-free sensor system. With magnet-based angular displacement systems, in particular with multi-turn absolute value encoders, a rotation of a shaft is inductively detected by a measuring unit, the measuring unit in particular comprising a rotating exciter unit, such as a permanent magnet, and a stationary sensor unit with at least one sensor, such as, for example, a Hall and/or a Wiegand sensor. The measuring unit is thereby mostly arranged at the free end of the shaft to be monitored.
Slight measuring errors frequently occur, however, when arranging or mounting a magnet based angular displacement measuring system directly on a drive shaft, in particular on a drive shaft of an electric motor or an electric generator. Such measuring errors are most often caused by interferences acting on the angular displacement measuring system from outside. Examples of such interference include a magnetic field caused by the drive shaft being magnetized in use by the electric motor or by an electromagnetic brake, and the magnetic field being transferred via the shaft which is typically made of steel so that, at the angular displacement measuring system, the rotational magnetic field formed by the exciter unit eventually changes, thereby causing measuring errors in the sensor unit. It is therefore necessary to avoid such interferences in the angular displacement measuring system in order to improve measuring accuracy.
DE 38 13 610 A1 describes an angle measuring means with a scanning means, wherein the scanning means is shielded from electric disturbing influences. The scanning means is thereby fastened in a housing in an electrically insulated manner and is connected to the mass potential of an evaluation unit. The housing is also in electric contact with the drive unit so that the interference signals outputted by the drive unit do not negatively affect the measuring values.
A drawback is, however, that magnetically induced interferences are not shielded off and measuring errors are still caused in the sensor unit, in particular with a magnet-based angle measuring means. The means for shielding the angle measuring means also has a very complex structure and comprises a great number of components.